1. Field of the Invention
The present invention relates to an improved punch assembly, including a punch, stripper spring assembly, and punch guide for use in a punching machine.
2. Prior Art
In high speed punching machines, it is desirable to maintain the shortest punching stroke possible because a long punching stroke increases the time required to punch a hole, thereby decreasing the productivity of the punching machine. In addition, the longer stroke causes greater compression of the stripper spring, which shortens its life because it is subjected to greater flexing. To shorten the punching stroke, it is desirable to maintain the top of the punch assembly as close to the ram as possible, and to maintain the extreme bottom end, that is, the cutting end of the punch, as nearly flush with the bottom of the punch guide as possible. At the same time, the overall length of the entire punch assembly must be maintained within small tolerances to keep the top of the punch assembly from striking the ram laterally as the turret rotates.
After a period of use, the cutting end of the punch wears and must be sharpened by grinding. Sharpening shortens the punch, which undoes the flush relationship between the cutting end of the punch and the end of the punch guide. To reestablish the flush relationship, the punch must be sharpened and readjusted to proper length.
When great numbers of punches are employed in many punching machines, much time and labor must be expended to sharpen and otherwise maintain the punches. Punches that can be sharpened only once or twice, as is the case in nearly all the prior art, must be discarded after a relatively short useful like. Also, the punch and stripper spring assemblies of the prior art are attached together in a manner that requires complete disassembly of the stripper spring assembly in order to remove the punch. Disassembly of the stripper spring assembly is tedious, time consuming, and frequently requires the use of special tools.
The prior art attempts to solve the above disadvantages have not been entirely successful. For example, in U.S. Pat. No. Re 29,958 issued to Cady on Apr. 10, 1979, a punch assembly is disclosed having a punch with an adjustable length through use of a reversible split ring retainer having a lip on one side. The split ring retainer is placed between the stripper string and a fixed shoulder on the punch. Before the punch has been sharpened, the side of the split ring retainer without the lip bears against the shoulder of the punch; the lip extends into an enlarged bore in a washer mounted on the other side of the retainer to effectively eliminate the width dimension of the lip. After the punch has been sharpened, the split ring retainer is reversed so the lip effectively compensates for the ground off portion of the punch.
The above technique for adjusting the effective length of the punch has several disadvantages. The required disassembly of the punch and stripper spring assembly to reverse the split ring retainer is an inefficient and cumbersome procedure. The lip of the split ring retainer also is very fragile and must be short to sustain punching forces, thus severely limiting the amount the punch can be lengthened. In addition, the punch can be sharpened only once and the amount of adjustment is fixed according to the dimension of the short lip, which may not correspond precisely with the length lost through sharpening.
In U.S. Pat. No. 4,141,264 issued to Weisbeck on Feb. 27, 1979, a punch assembly is disclosed that permits the punch to be sharpened a greater number of times to prolong the life of the punch. Two adjustments are required. First, after sharpening, one or more retainer shims approximately 1/16th of an inch thick are driven over the bottom portion of the punch until they seat above a shoulder in the punch. Second, the position of the head of the stripper spring assembly, which is threaded onto the top end of the punch, is adjusted on the punch. These two adjustments maintain the same effective length of the punch and the same tension on the stripper spring. This technique for adjusting the effective length of the punch is, however, awkward and cumbersome. Adjustments can be made only in discrete amounts and the retainer shims are not sturdy enough to withstand sustained punching. The amount of adjustment is limited by the weakness of multiple retainer shims. Also, replacement of a punch requires complete disassembly of the punch assembly. Finally, the critical overall length of the punch assembly requires exacting readjustment of the length of the stripper spring assembly. If the stripper spring assembly is only slightly too long, it will strike the ram laterally when the turret rotates, which can severely damage the punching device.
Finally in U.S. Pat. No. 2,355,344 issued to Wales on Sept. 10, 1935, a punch assembly is disclosed having a punch whose length is adjustable by turning nuts threaded onto the top portion of the punch. These nuts are separate from the stripper spring assembly so the punch itself is not fastened or attached to the stripper spring assembly. By providing a punch that is not fastened to the stripper spring assembly, the punch can be easily removed for sharpening and the central stripper spring, which is not a fixed part of the stripper spring assembly, can be easily replaced. The Wales punch requires a bulky and cumbersome stripper spring assembly which severely limits the usefulness of this punch and stripper spring assembly in modern high speed punching machines. Furthermore, since the punch is not fastened to the stripper spring assembly, it would vibrate excessively when subjected to the rapid and sustained impact of a modern punch press.